1. Introduction
This invention is directed to the manufacture of articles having an imaged surface and more particularly, is directed to a process for applying a light sensitive photoresist material in a novel manner in the manufacture of such articles.
2. Description of the Prior Art
Resists are used in the manufacture of numerous items to provide a relief image over a substrate. One of the largest uses of resists is in the manufacture of printed circuits where the circuit is typically first defined by a resist image of the desired circuit over a suitable substrate such as a phenolic or glass-filled epoxy substrate.
In the manufacture of printed circuits, primarily two types of resists are used. For low cost printed circuits, resists are used that are not light sensitive and the relief image is provided by application of the resist over the substrate directly in the image pattern. For those printed circuits requiring greater precision and detail and where higher cost is justified, light sensitive resists are used where the resist is applied as a continuous layer and the relief image is formed in the layer by exposure to a source of activating radiation through a master followed by chemical development of the exposed layer to provide the relief image.
Resists which are not light sensitive are applied to a circuit board substrate material in a relief image pattern, most frequently by screening through a stencil. The technique of screen printing is particularly adaptable to low cost printed circuits where the ultimate in resolution and definition is not required. Basically, the process involves a screening operation which depends upon the transfer of a resist, frequently to a copper-clad laminate surface, using a stencil where the openings in the stencil define the circuit pattern. The stencil is firmly attached to the surface of a silk, nylon or stainless steel screen stretched drum-head tight over a frame suspended a short distance above the surface of the substrate. Resist is forced through the open areas of the screen and onto the laminate by the wiping motion of a rubber squeegie. This results in the resist defining the desired circuit pattern. The remaining steps for the formation of a printed circuit comprise drying the resist, and where the resist has been applied over a copper-clad substrate, etching the exposed copper whereby the copper remaining defines the circuit pattern. Details of screening resists over a substrate in the manufacture of printed circuits can be found in Coombs, Printed Circuit Handbook, McGraw-Hill Book Company, New York, 1967, pages 4-25 to 4-36; Scarlett, Printed Circuit Boards For Microelectronics, Van Nostram-Reinholdt Company, London, 1970, pages 39 and 40; Draper, The production of Printed Circuits and Electronic Assemblies, Robert Draper Ltd., Teddington, October, 1969, pages 57-60; and Schlabach, Printed and Intergrated Circuits, Materials and Processes, McGraw-Hill Book Company, New York, 1963, pages 83-89; all incorporated herein by reference.
There are cost advantages to the use of screen resists. The resists are low cost materials which do not require special care in their use other than ordinary cleanliness. The screen material used is inexpensive and the preparation of a screen bearing the desired circuit image is a low cost process. Once the screen is formed, it can be repeatedly reused for printing multiple substrates. The screening process may be performed by relatively untrained technicians or may be performed using automated equipment. The coatings obtained are uniform and of good quality. Finally, from the standpoint of material consumption, notwithstanding that the screen resists are relatively low cost materials, there is little waste of material in the screening process, thus resulting in even greater cost efficiency.
Offsetting the cost advantages enumerated above, screen resists cannot be used for the manufacture of printed circuits where precision and definition are required. When the resist is squeezed through the screen under pressure, the pressure causes the resist to flow both through screen openings and beneath the screen resulting in spreading of the lines. In addition, with use, the screens stretch and successive substrates coated with screen resists are out of registration with each other. As a consequence, fine line circuit definition is unavailable using screening processes. Where precision and definition are required, the industry uses light sensitive resists. These materials are capable of being imaged and developed with photographic resolution.
Photoresists may be negative or positive acting materials. Both comprise light sensitive materials in a resin binder. If the resist is a negative acting resist, the exposed areas are hardened by light exposure whereas, positive resists are solubilized by such light exposure. The various types of photoresists available and their formulations are set forth in DeForest, Photoresist, Materials and Processes, McGraw-Hill Book Company, New York, 1975, pages 4 and 5 and 19 through 61; and Printed Circuit Handbook, supra, pages 4-11 through 4-17, both incorporated herein by reference.
There are several methods for applying photoresists to circuit board substrate materials. The most common wet methods comprise dip coating, whirl coating, roller coating, curtain coating and spray coating.
Dip coating comprises immersing a substrate to be coated into a tank containing photoresist. It is an easy and quick way of coating both sides of a substrate, but after the coated substrate is removed from the tank and as the photoresist drains from the substrate, there is a tendency for streaks to form resulting in coatings of uneven thickness and therefore, incomplete or uneven development may occur. Moreover, with through hole substrates, there is a tendency for the resist to run into and plug the through holes.
Whirl coating utilizes centrifugal force to distribute the resist over an area to be coated. The coatings have a more uniform thickness than coatings obtained by dip coating though there is a tendency for the coating to be thinner in the center than along the edges. A major disadvantage of whirl coating is encountered with double-sided boards, especially those having through holes. Resist tends to run around the edge or through the holes causing streaks on the reverse side, again resulting in incomplete or uneven development. Another disadvantage is substantial loss of resist during the coating process as it is spun off the edge of the substrate and generally, more than fifty percent of the resist is lost.
All photoresists can be roller coated except those of low viscosity. Roller coaters utilize rollers having soft surfaces such as surfaces of rubber that are grooved or knurled whereby the roller can act as a reservoir for the resist. A major problem with roller coating arises from spreading the resist over the large surface of the roller in a relatively thin cross section. This results in excessive exposure of the resist to air which can degrade the resist, particularly negative acting resists which are frequently oxygen sensitive.
Spray coating can use common paint spray equipment and practically all resists can be spray coated. Major objections to this process are high cost due to over-spray resulting in an uneven coating and wasted resist and excessive solvent emissions due to the high solvent content necessary to provide resists of sufficiently low viscosity for spraying.
From the above, it is apparent that screening is a method for applying resists which are not light sensitive to obtain uniform coatings without wasting large volumes of resist, but this process cannot be used where fine image resolution is required. Alternatively, the methods associated with the application of light sensitive resists provide fine image resolution, but coatings of uniform thickness are difficult to obtain and considerable resist is wasted in the process significantly increasing the cost of manufacture.